1. Technical Field
The disclosure relates to a method for checking a light amount, and more particularly to a method for compensating and checking a light amount of a light-emitting device.
2. Related Art
A photocopier, a printer, a fax machine, and a multifunction printer adopt electro-photography as the core technology for printing documents, that is, a photographic image is produced by changing an electrostatic charge with light with a specific wavelength.
Please refer to FIG. 1, in which a schematic view of a color light-emitting diode (LED) printer 100 is shown. The LED printer 100 includes photoconductive drums (110K, 110M, 110C, and 110Y, generally referred to as 110), printing heads (120K, 120M, 120C, and 120Y, generally referred to as 120) and toner cartridges (130K, 130M, 130C, and 130Y, generally referred to as 130), corresponding to black, magenta, cyan and yellow, respectively. An even layer of charges is produced on the surface of the photoconductive drum 110 through a power distribution mechanism. Before printing, an exposure procedure is required in a scanning procedure, so that image pixels in the document to be printed are converted into bright and dark data of visible light. The printing head 120 has a plurality of LEDs. When the light emitted from the LEDs is projected on the photoconductive drum 110, original potential is maintained in unexposed areas, while charges in exposed areas vary due to the exposure. Due to the potential differences of exposed areas, toner with a positive/negative charge provided in the toner cartridge 130 may be attracted, so as to achieve the printing objective.
FIG. 2 is a view illustrating a relationship between a printing concentration and exposure energy accepted by the photoconductive drum. As shown in FIG. 2, the printing concentration is positively correlated to the exposure energy of the photoconductive drum. When the exposure energy accepted by the photoconductive drum increases, the printing concentration also increases, thereby printing document content with different gray scales.
FIG. 3 is a schematic outside view of the printing head 120 of the LED printer 100. As shown in FIG. 3, the printing head 120 includes a plurality of light-emitting chips 122 arranged along an axis 140. Generally speaking, each light-emitting chip 122 includes thousands of linearly arranged LEDs. When the light-emitting chips 122 are arranged along the axis 140, the LEDs are also arranged along the axis 140, so as to achieve high DPI (Dots Per Inch) printing resolution. For example, to achieve a DPI resolution of 1200×2400, 1200 LEDs must be arranged in every inch.
However, to achieve even concentration of the printed document, the light amount output by each LED in the printing head 120 must be controlled precisely, so as to avoid excessive exposure or inadequate exposure of exposed areas of the corresponding photoconductive drum 110. Since the light-emitting characteristics of the LEDs are different from each other, each light-emitting chip 122 has to be tested and calibrated before being mounted in the printing head 120. Each printing head 120 includes a large number of LEDs and each color LED printer 100 further includes four printing heads 120. Therefore, the approach to achieve efficient test and calibration is a subject to which researchers in the art dedicate themselves.